1. Field of the Invention
This invention relates to multi-step light-emissive devices and light-emissive organic materials.
2. Related Technology
An emerging class of display devices uses an organic material for light emission. Light-emissive organic materials are described in PCT WO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emissive organic layer, for instance a film of a poly(p-phenylenevinylene (“PPV”), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes combine in the organic layer, generating photons. In PCT WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinolino)aluminium (“Alq3”). In a practical device, one of the electrodes is typically transparent, to allow the photons to escape the device.
FIG. 1 illustrates the cross-sectional structure of a typical organic light-emissive device (“OLED”). The OLED is typically fabricated on a glass or plastic substrate 1 coated with a transparent first electrode 2 such as indium-tin-oxide (“ITO”). Such coated substrates are commercially available. This ITO-coated substrate is covered with at least a layer of a thin film of an electroluminescent organic material 3 and a final layer forming a second electrode 4, which is typically a metal or alloy. Other layers can be added to the device, for example to improve charge transport between the electrodes and the electroluminescent material.
Typically, the film 3 of organic material is around 60 to 100 nm thick. At these thicknesses the turn-on voltage for an OLED of the type shown in FIG. 1 is normally around 2V. Peak efficiency generally occurs at an applied voltage in the range from 2 to 6V. Once the voltage drop across the OLED is high enough to inject both carrier types satisfactorily into the device any additional voltage causes additional light to be emitted, but at a lower efficiency since extra work is done in transporting the carriers through the polymer at the higher electric field. For thicker devices the efficiencies are also lower because extra work has to be done in transporting the carriers further. At significantly higher applied voltages the device breaks down.
It would be desirable to use OLED technology for general lighting applications, such as domestic and commercial building lighting and external lighting such as street lighting. To do this efficiently it would be desirable for OLED devices to be capable of being driven efficiently from higher voltages than are currently employed for OLEDs: preferably by mains voltages of 220V or 110V, or higher.
One way to achieve this would be to form a stacked device by laminating together a number of devices of the type shown in FIG. 1, so that the cathode of one device feeds current to the anode of the next. This arrangement is illustrated in FIG. 2, in which like parts are numbered as for FIG. 1. If, say, 4V were dropped across each device in the stack, and the stack comprised 50 devices then 200V would be dropped across the entire stack. However, a device of this sort has a great number of electrode layers. Therefore, it is very difficult to arrange for the electrodes to be sufficiently transparent that they do not block a great deal of the light that is emitted by the devices of the stack. This is a major problem in building an efficient device to this architecture.
FIG. 3 shows the structure of a dispersed-type inorganic electroluminescent device. A device of this type uses an entirely different principle to emit light. Phosphor particles 10, for example comprising ZnS, are dispersed in a transparent conductive matrix 11 between anode and cathode electrodes 12, 13. When a suitable voltage is applied between the electrodes electrons are lifted from deep traps in the phosphor particles and accelerated to high energy. The high-energy electrons can then collide with light-emitting centres at the particles, where ionisation takes place. Thus, in a device of this type the emission is from the phosphor particles, not the matrix.